1. Field of the Invention
An aspect of the present invention relates to a radio system, radio apparatus, and an antenna device, and more particularly to an antenna device including a loop-type antenna element and radio apparatus and a radio system using the antenna device.
2. Description of the Related Art
An individual identification technique utilizing radio communication (Radio Frequency Identification which will be abbreviated hereinafter as “RFID”) is widely utilized for automatic ticket gates of a railway, management of times of arrival and departure of employees at and from a corporation or an office, various types of electronic money, and the like. In the RFID, information is exchanged through radio communication between a device (reader/writer) and an information medium (card or tag). A loop antenna built into the reader/writer and a loop antenna built into the card are held in a communicable state while being opposed each other, whereupon the reader/writer can write information into the card or read information from the card.
Some types of portable cellular phones are equipped with a function compatible with such RFID. In recent years, the portable cellular phones have not only the card function but also the reader/writer function. There is a strong request for miniaturization of the reader/writer including a portable cellular phone. However, there is a problem of a metallic portion of the device intended for miniaturization coming close to a loop antenna, thereby generating an eddy current induced by an A.C. magnetic field and shortening a communicable distance between the reader/writer and a card.
A technique of preventing generation of such an eddy current by utilization of a magnetic substance has been disclosed (see, e.g., JP-A-2007-122225 or JP-A-2006-178713). In a non-contact IC card reader described in JP-A-2007-122225, the decorative laminate panel is attached to the mount frame (made of metal), which is used to mount the module onto the housing, through a base. A loop antenna is laminated on the base by through the magnetic substance layer, to thus shield a space between the antenna and the mount frame with the magnetic substance layer and prevent occurrence of an eddy current in the mount frame.
An information processing device described in JP-A-2006-178713 has a structure in which a conductive plate, a magnetic substance, and an antenna substrate are sequentially arranged on a circuit substrate in a layered manner, thereby shielding the circuit substrate from a magnetic flux of the antenna and preventing occurrence of an eddy current.
In the RFID, loop antennas built into the reader/writer and the card constitute respectively resonators, and nominal values of resonance frequencies of the resonators are set equally to each other. In general, it is known that, when two resonators having the same resonance frequency come close to each other, frequencies of the two resonators are gradually separated toward two frequencies f1 and f2 (f1<f2) (for example, Kawaguchi et al., “Study of Equivalent Circuit Display of Electromagnetic Coupling between Distributed Constant Resonators”, Technical Research Report EMCJ2003-78/MW2003-175 of the IEICE, October, 2003; and Ito et al., “Relationship between a Dead Zone and a Coupling Coefficient in an HF band RFID”, General Convention B-1-143 of the IEICE, March, 2007). This phenomenon is called a frequency split. The frequency split arises when strong coupling occurs as a result of a space between the reader/writer and the card being reduced to a certain extent or more. When the value of the frequency split increases in excess of a limit, it may be the case where communication cannot be established between the reader/writer and the card.
The frequency split is described by reference to FIGS. 11 through 13 while taking the RFID system as an example. FIG. 11 is an exemplary measurement result of a frequency characteristic of a return loss of a card for an RFID system. A horizontal axis in the drawing represents a frequency; the center of the plot corresponds to 13.56 megahertz (MHz); and one tick mark corresponds to 1 MHz. A vertical axis in the drawing represents a return loss; the maximum value represents 0 dB; and one tick mark corresponds to 1 dB.
FIG. 12 shows an exemplary measurement result of a frequency characteristic of a return loss of a related-art reader/writer for an RFID system. A horizontal axis in FIG. 12 is identical with the horizontal axis shown in FIG. 11. A vertical axis in FIG. 12 represents a return loss; the maximum value corresponds to 0 dB; and one tick mark corresponds to 0.2 dB.
FIG. 13 is an exemplary measurement result of frequency characteristic of return losses performed when the loop antenna of the card whose characteristic is shown in FIG. 11 and the loop antenna of the reader/writer whose characteristic shown in FIG. 12 are brought closely to each other in a mutually-opposing manner. The horizontal axis and the vertical axis shown in the drawing are identical with their counterparts in FIG. 12. In the drawings, a left resonance point represents a resonance point of the card, and a right resonance point represents a resonance point of the reader/writer. In this case, the value of the foregoing frequency split has come to about 5.5 MHz, whereupon the frequency split manifests itself noticeably. Under such conditions, it is extreme difficult to perform radio communication between the card and the reader/writer.
The related-art technique described in JP-A-2007-122225 is intended for assuring a communicable distance by means of preventing occurrence of an eddy current, which would otherwise be arise in a metal mount frame, to thus lessen an adverse effect on an antenna characteristic. However, no consideration is given to the problem of the frequency split induced as a result of the reader/writer coming closely to the card.
The related-art technique described in JP-A-2006-178713 is directed toward preventing a magnetic flux from reaching a substrate by converting a magnetic flux leaked from an antenna into an eddy current by means of a conductive plate, thereby preventing occurrence of an eddy current also. However, no consideration is given to the problem of the frequency split induced as a result of the reader/writer coming closely to the card.